Hitherto, the maintenance inside a nozzle stub of a reactor vessel has been necessarily performed under water since the inside of the reactor vessel is normally filled with cooling water. However, in order to perform the maintenance under water, all devices need to be designed to be waterproof. For this reason, since the devices are designed to be waterproof, the volume and the weight of the device increase, so that the handling of the device is difficult. For this reason, recently, a method of performing the maintenance inside the nozzle stub of the reactor vessel in atmosphere has been proposed.
Specifically, a working platform is installed inside the reactor vessel after the water level of the cooling water therein is made to be lower than that of the nozzle stub. Inside the working platform, a maintenance worker inserts and fixes a UT inspecting device into the nozzle stub, and performs UT inspection on a desired range.